Many vehicles are used over a wide range of vehicle speeds, including both forward and reverse movement. Some types of engines, however, are capable of operating efficiently only within a narrow range of speeds. Consequently, transmissions capable of efficiently transmitting power at a variety of speed ratios are frequently employed. When the vehicle is at low speed, the transmission is usually operated at a high speed ratio such that it multiplies the engine torque for improved acceleration. At high vehicle speed, operating the transmission at a low speed ratio permits an engine speed associated with quiet, fuel efficient cruising. Typically, a transmission has a housing mounted to the vehicle structure, an input shaft driven by an engine crankshaft, and an output shaft driving the vehicle wheels, often via a differential assembly which permits the left and right wheel to rotate at slightly different speeds as the vehicle turns.
Some vehicles are equipped with a transfer case which directs the power to both front wheels and rear wheels. Some transfer cases provide multiple transfer case ratios between the transmission output shaft and the differential such that a driver can select a high range and a low range. The high range may be selected for on-road transportation while the low range may be used to provide higher speed ratios for off-road use. When a two speed transfer case is present, the overall ratio is the product of the transmission ratio and the transfer case ratio. In some situations, such as transitioning from on-road to off-road or from off-road to on-road conditions, it is desirable to shift between high and low range while the vehicle is moving, preferably without interrupting the flow of power to the vehicle wheels. In a transverse front wheel drive vehicle, space limitations usually preclude use of a two speed transfer case.